s390x/tcg: Implement 64 bit for VECTOR FP LOAD LENGTHENED

64 bit -> 128 bit, there is only a single final element.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210608092337.12221-18-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>